Many chemicals enter the bodies of animals, both human and nonhuman, in various manners, for example, across skin, by inhalation, by oral ingestion and by direct injection through the skin into subcutaneous tissues and muscle or directly into blood vessels. These chemicals are distributed in the body via blood circulation. Owing to concentration gradients, a small portion of the circulating chemicals, or one or more of their metabolites, may migrate outwardly from skin capillaries, across the epidermis and stratum corneum to the skin surface. If the thus migrated chemicals or metabolites are vaporous at the skin surface temperature, they will escape from the skin surface into the adjacent environment. Additionally,, gases such as oxygen and carbon dioxide in capillary blood also diffuse to the skin surface and escape.
Various methods and apparatuses are known in the art for assessing human exposure to chemicals by monitoring and/or collecting substances which migrate to a subject's skin surface. For example, Applicant's copending application Ser. No. 660,778 discloses a dermal substance collection device for the transdermal collection of non-volatile chemicals. However, the device disclosed therein includes a fluid conduit between the skin surface and the binding component, a feature which is counterproductive for the collection of chemicals in the vapor state. Similarly, the Phillips U.S. Pat. No. 4,595,011 discloses a transdermal dosimeter for monitoring exposure to chemical agents which includes a fluid phase serving as a dermal contact bridge.
Peck et al in "A Non-Invasive Transepidermal Toxicological Monitoring Device", Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy", Mar. 9, 1982, Meeting Abstracts Book, page 366, disclose a transepidermal device including an adhesive plastic chamber containing sodium chloride-impregnated absorbent pads and an activated charcoal impregnated disk for capturing a xeobiotic to prevent back transfer from the collection device into the body. Similarly, the Phillips U.S. Pat. No. 4,329,999 discloses a patch including sodium chloride-impregnated absorbent pads for absorbing sweat from the skin surface of a patient. Sodium chloride interferes with the collection of chemicals which are in the vapor state.
The Delpy et al U.S. Pat. No. 4,396,017 discloses a transcutaneous gas sensor including a sensing electrode which is applied to the boyy surface of a patient with an electrolyte layer between the electrode and the patient's body. Gas which has diffused through the patient's body from his blood to the body surface results in an electrochemical reaction at the exposed surface of the electrode. Similarly, the Dali et al U.S. Pat. No. 4,280,505 discloses a transcutanoous gas sensor probe for the measurement of trancutaneousggases emitted through the skin of a living body.
Additional methods are also known for determining the presence of various chemicals in the body. For example, tissue biopsies and blood sampling are used but are disadvantageous owing to their invasive character. Other methods such as collection of urine or saliva are disadvantageous owing to the difficulty of obtaining continuous monitoring. A need exists for a method and apparatus for the non-invasive and continuous collection of substances which transdermally migrate to and vaporize at the surface of the skin of a subject.